drm/vc4: Call drm_atomic_helper_fake_vblank() in the commit path

Mimic what is done in drm_atomic_commit_tail() and call
drm_atomic_helper_fake_vblank() so that VBLANK events are faked
when the drm_crtc_state.no_vblank is true. Will be needed when we'll
add support for the transposer block.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-8-boris.brezillon@bootlin.com